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Do I need to place an expansion joint at these doorways?
IMO, no. Your using quartersawn. That makes a big difference. Acclimate properly and maintain temps and RH at normal, living conditions and that floor won't move.
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Could I instead start laying the floor from the middle of the house (using a slip tongue) and work my way out?
Yes. And it maybe the most efficient start point.
I've laid 3/4" x 3&1/4" plain sawn over greater spans with no problems. The key is moisture control, temp control and humidity control.